K-64 is simply 4x K-16 period. They even have connectors for this very purpose fitted onto the K-16 reference design.
Yes, you can "hook" the K-16 boards together in an array. However, The developer BKKCoins himself has said that he will start work on the K64 board as soon as the K-16 is complete.
https://bitcointalk.org/index.php?topic=190731.0